This invention relates to gyroscopes in general and more particularly to an improved ring laser gyroscope cluster.
The conventional inertial measurement unit employing ring laser gyroscopes uses individually body dithered ring laser gyroscopes mounted to a common elastically supported structure, called the stable element. Usually three orthogonally mounted ring laser gyroscopes are used, along with strapped down accelerometers, attached rigidly to the stable element. Analysis shows that the stable element is rotationally vibrated violently about a diagonal axis in space in reaction to the moments imposed on it by the dithered ring laser gyroscopes. The mechanical coupling between the ring laser gyroscopes is so strong as to practically rule out any other dither but phase locked motion. Any other motion would cause the common reaction moment vector to deviate from a fixed space diagonal. If the three strapped down accelerometers are subjected to such a condition, they sense not only the rotational acceleration of about 500 rad/sec.sup.2, but also a linear vibrational acceleration that may amount of 5.2 g for a 4 inch offset, both at the dither frequency, typically 250 to 450 Hz. Such simultaneous rotational and linear acceleration is particularly harmful in that it may cause a rectification in the accelerometers that will vary as the offset varies. A changing bias may thus result. For a strapped down accelerometer, a typical specification is bias stability in the order of 5-10.sup.-6 g. This is indeed a difficult problem when the noise to signal ratio is 1,000,000:1.
In order a facilitate a lower noise to signal ratio, the ring laser gyroscopes with individual dither can be phase locked within tight tolerances, and the symmetry of the inertia axis and suspension of the stable element can be closely controlled and adjusted. In addition, the inertia, spring rate and natural frequency of the individual ring laser gyros can be closely controlled and adjusted so that the available torque capacity of each dither motor is not overtaxed. Provided the adjustments are correctly performed, it is then possible to utilize an accelerometer mounting scheme in which only a rotational vibration input is sensed by the strapped down accelerometers. The accelerometers must still perform in this rotational vibration environment. Each ring laser gyroscope must also be statically balanced for linear acceleration inputs at the dither frequency, perpendicular to its input axis, in two mutually perpendicular directions.
Each ring laser gyro also contain electronics, compensating beam combiners, dither suspension and dither drive motors.